On 09/05/2023 18.27, Claudio Imbrenda wrote:
When rebooting a small VM using asynchronous teardown, a spurious
warning is emitted when the KVM_PV_ASYNC_CLEANUP_PREPARE ioctl fails.
Why does the _PREPARE fail in that case? Why 4GiB and not more or less? This
sounds racy... what if you have a faster or slower machine?
Avoid using asynchronous teardown altogether when the VM is small
enough; the cutoff is set at 4GiB. This will avoid triggering the
warning and also avoid pointless overhead; normal teardown is fast
enough for small VMs.
Reported-by: Marc Hartmayer <mhart...@linux.ibm.com>
Fixes: c3a073c610 ("s390x/pv: Add support for asynchronous teardown for reboot")
Signed-off-by: Claudio Imbrenda <imbre...@linux.ibm.com>
---
hw/s390x/pv.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
index 49ea38236c..17c5556319 100644
--- a/hw/s390x/pv.c
+++ b/hw/s390x/pv.c
@@ -13,6 +13,7 @@
#include <linux/kvm.h>
+#include "qemu/units.h"
#include "qapi/error.h"
#include "qemu/error-report.h"
#include "sysemu/kvm.h"
@@ -117,13 +118,16 @@ static void *s390_pv_do_unprot_async_fn(void *p)
bool s390_pv_vm_try_disable_async(void)
{
+ MachineState *machine = MACHINE(qdev_get_machine());
The calling function (s390_machine_unprotect()) already has a
S390CcwMachineState as parameter ... so you could pass along that value to
avoid the qdev_get_machine() here.
/*
* t is only needed to create the thread; once qemu_thread_create
* returns, it can safely be discarded.
*/
QemuThread t;
- if (!kvm_check_extension(kvm_state, KVM_CAP_S390_PROTECTED_ASYNC_DISABLE)) {
+ /* Avoid the overhead of asynchronous teardown for small machines */
+ if ((machine->maxram_size < 4 * GiB) ||
+ !kvm_check_extension(kvm_state, KVM_CAP_S390_PROTECTED_ASYNC_DISABLE))
{
return false;
}
if (s390_pv_cmd(KVM_PV_ASYNC_CLEANUP_PREPARE, NULL) != 0) {
Thomas
